Focusing the Finder Scope

1Choose a target that is in excess of one mile away. This eliminates any possible parallax effect between the tele- scope and finder.

2Release the altitude and azimuth clamps of your telescope mount and point the telescope at your target.

3Center your target in the main optics of the telescope. You may have to move the telescope slightly to center it.

4Now look in the finder scope. Is the object visible? Ideally it will be somewhere in the field of view. If not, some coarse adjustment to the finder scope bracket’s alignment thumb- screws will be needed until the object comes into the finder scope’s field of view.

5.With the image in the finder scope’s field of view, adjust the alignment thumbscrews one at a time to center the object on the intersection of the crosshairs.

is sharp. Once an image is in focus, turn the knob clockwise to focus on a closer object and counterclockwise for a more distant object. A single turn of the focusing knob moves the primary mirror only slightly. Therefore, it will take many turns (about 30) to go from close focus to infinity.

For astronomical viewing, out-of-focus star images are very diffuse, making them difficult to see. If you turn the focus knob too quickly, you can go right through focus without seeing the image. To avoid this problem, your first astronomical target should be a bright object (like the Moon or a planet) so that the image is visible even when out of focus. Critical focus- ing is best accomplished when the focusing knob is turned in such a manner that the mirror moves against the pull of grav- ity. In doing so, any mirror shift is minimized. For astronomical observing, both visually and photographically, this is done by turning the focus knob counterclockwise.

If, when you look through the finder scope, the images appear somewhat out of focus, you will need to refocus the finder scope for your eyes.

1.Loosen the lock ring located behind the objective lens cell on the body of the finder scope (see Figure 3b).

2.Back the lock ring off by a few turns, for now. Refocus the finder scope on a distant object by threading the objective lens cell in or out of the finder scope body. Precise focusing will be achieved by focusing the finder scope on a bright star.

3.Once the image appears sharp, retighten the locking ring behind the objective lens cell. The finder scope’s focus should not need to be adjusted again.

Image orientation through the finder scope is upside down.This is normal for any finder that is used straight-through. Because of this, it may take a few minutes to familiarize yourself with the directional change each screw makes on the finder.

Focusing the Telescope

Image Orientation

The image orientation changes depending on how the eye- piece is inserted into the telescope. When using a star diago- nal, the image is right-side-up but reversed from left to right (i.e., mirror image). If inserting an eyepiece directly into the visual back (i.e., without a star diagonal), the image is upside- down and reversed from left to right (i.e., inverted). This is nor- mal for the Schmidt-Cassegrain design.

Normal image	With star diagonal,	With an eyepiece
orientation as seen	image in telescope	but no diagonal,
with unaided eye.	is reversed.	image in telescope
		is inverted.

Figure 5. Image orientation in Schmidt-Cassegrains

Calculating Magnification

You can change the magnifying power of your telescope by changing the eyepiece (ocular). To determine the magnifica-

The Schmidt-Cassegrain focusing mechanism con- trols the primary mirror, which is mounted on a ring that slides back and forth

tion of your telescope, simply divide the focal length of the tele- scope by the focal length of the eyepiece used. In equation format, the formula looks like this:

on the primary baffle tube. The focusing knob, which moves the primary mirror, is on the rear cell of the

Telescope Focal Length (mm)

Eyepiece Focal Length (mm)

= Magnification

telescope. To focus, turn the focusing knob until the image is sharp. If the knob will not turn, it has reached the end of its travel on the focusing mechanism. Turn the knob in the opposite direction until the image

Figure 4. The emblem on the end of the focus knob shows the correct rotation direction for focusing your telescope.

For example, if you are using a 26mm eyepiece in the 8" Schmidt-Cassegrain, divide the focal length of the telescope (2032mm) by the focal length of the eyepiece, 26mm. That yields a magnification of 78.

Although the power is variable, each instrument has a limit to how much it can practically achieve. The general rule is 60x magnification per inch of aperture. So an 8"-aperture telescope can achieve a maximum useful magnification of 480 power.